From TARDIS Index File, the free Doctor Who reference.

Dunkirk was a port city in northern France.

During World War II, from late May to early June 1940, thousands of British Army soldiers were evacuated from the area after they had been cut off from the rest of the army by the German advance.

Although the German army could have easily destroyed the British forces, the Doctor had convinced Adolf Hitler, who was unstable and disoriented after the Timewyrm had been banished from his mind, to not advance on Dunkirk. (NA: Timewyrm: Exodus)

The true reason why the Germans did not attack Dunkirk remains unknown.

The Doctor had been present at Dunkirk. (NA: Just War, PDA: Byzantium!)

The Doctor and Donna Noble prevented a Nemonite from taking possession of those involved in the Dunkirk evacuation. (The Nemonite Invasion)

In an alternate timeline in which Hitler kept the power of the Timewyrm, the Germans advanced on Dunkirk and totally defeated the British Army. (NA: Timewyrm: Exodus)

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Dunkirk